HIGH-RESOLUTION CROSSWELL IMAGING OF A WEST TEXAS CARBONATE RESERVOIR.3. WAVE-FIELD SEPARATION OF REFLECTIONS

Using crosswell data collected at a depth of about 3000 ft (900 m) in West Texas carbonates, one of the first well-to-well reflection images of an oil reservoir was produced. The P and S brute stack reflection images created after wavefield separation tied the sonic logs and exhi bited a vertical resolution that was comparable to well log resolution . Both brute stacks demonstrated continuity of several reflectors know n to be continuous from log control and also imaged an angular unconfo rmity that was not detected in log correlations or in surface seismic profiling. The brute stacks, particularly the S-wave reflection image, also exhibited imaging artifacts. We found that multichannel wavefiel d separation filters that attenuated interfering wavemodes were a crit ical component in producing high-resolution reflection images. In this study, the most important elements for an effective wavefield separat ion were the time-alignment of seismic arrivals prior to filter applic ation and the implementation of wavefield-separation filters in multip le domains, particularly in common offset domain. The effectiveness of the multichannel filtering was enhanced through the use of extremely fine wellbore sampling intervals. In this study, 2.5 ft (0.76 m) verti cal sampling intervals for both source and receiver were used, whereas most previous crosswell data sets were collected with much coarser sa mpling intervals, resulting in spatial aliasing and limiting the utili ty of the data for reflection processing. The wavefield separation tec hniques employed in this study used data volumes and associated filter ing operations that were several orders of magnitude larger than those encountered in conventional VSP data analysis.